I. Field of the Invention
This invention relates to sealed bearing rotary cone rock bits and a means to lubricate opposing thrust bearing surfaces.
More particularly this invention relates to an improved method to distribute lubricant in the thrust surfaces formed in a rotary cone and the end of a main bearing of a rotary cone rock bit.
II. Description of the Prior Art
It is known to provide chordally disposed lube slots in an axial thrust surface formed on the end of a main radial journal bearing for a rotary cone rock bit. The chordally disposed slots are formed in the axial thrust surface of the leg. These generally chordally disposed slots leave sharp edges. These edges tend to remove lubricant from the opposing thrust surfaces rather than distribute lubricant on these surfaces. The corners of the slots are deburred by secondary manufacturing operations but still leave sharp edges.
U.S. Pat. No. 4,410,284, assigned to the same assignee as the present invention, teaches a rolling cutter drill bit with at least one downwardly extending leg which supports a cantilevered bearing shaft. A cutter cone is rotatably mounted on the bearing shaft. A radially disposed thrust washer is positioned between the journal and the cone, the washer further includes a lubricant transferring means in each radially disposed side of the washer to assure adequate lubrication of the roller cone mounted on the journal bearing. The thrust washer forms at least a pair of passages to transfer lubricant through the washer from one bearing surface to an opposite bearing surface. Grooves on opposite sides of the thrust washer distribute lubricant to the bearing surfaces. The grooves however have relatively sharp edges and tend to wipe an opposing bearing surface clean rather than distribute lubricant thereon.
The present invention provides a means to apply a film of lubricant on bearing surfaces by providing a ramp emanating from grooves in the bearing surface or shallow groove with rounded edges to create hydrodynamic pressures to facilitate distribution of the film of lubricant on the bearing surfaces.
Typically, the lubrication regime found on rock bits is assumed to be "Boundary Film Lubrication". This condition is where the bearing load is carried by surface asperities and lubrication is aided by chemical additives that modify the bearing material chemistry to form easily sheared compounds. This reduces friction but can increase wear. Typical contact bearing pressures range from 1,000 to 10,000 psi.
"Fluid Film Lubrication" is the condition where the load is carried entirely by a thin film of lubricant between opposing bearing surfaces. Heretofore, it was believed by those skilled in the rock bit art that the fluid film developed in rock bit bearings is thin enough that the condition meets the normally assumed criterion to be "boundary lubrication". Dudly D. Fuller, Stevens Professor Emeritus of Mechanical Engineering, Columbia University, teaches in, THEORY AND PRACTICE OF LUBRICATION FOR ENGINEERS, Second Edition, published in 1984 by JOHN WILEY & SONS, Inc., that the minimum allowable film thickness (for fluid film bearing) varies from 50 micro-inches to 3,000 micro-inches depending on the condition of the bearing materials and diameters.
Experimentation has shown that a substantial improvement in bearing performance occurs when the calculated film thickness is varied from 4.5 micro-inches to 35 micro-inches, much less than what is typically considered acceptable for fluid film. It was reasoned from this experimentation that the introduction of a mechanism which encourages the generation of a fluid film, albeit very thin, will significantly improve the friction and wear characteristics of the bearing of a seated bearing rotary cone rock bit.